(UDMA) Ultra Direct Memory Access is an interface type of the mobo. (ATA) Advanced Technology Attachment is an interface of the IDE device, in this case your hard disk. i beleive your hard disk will simply be "underclocked" to ATA66/33 from ATA100 speed.
i think this backwards compatability is inherited in most if not all of the ATA100/133 IDE hard disks out there. you should have no problems.